I'm running Debian Etch on a 4GB hard drive. I had plenty of disk space last year when I set the computer up, but lately the disk has been filling up. Today, I let the system automatically install 43 updates, and now I can only log in to GDM as root. When I log in as a regular user, GDM displays a message that it cannot write the authorization file and that my hard disk might be full.
I can open a Root Terminal. One time, the df command reported that /dev/hda1 had 3908554 1-K blocks. 3748072 were used 0 were available. Then I deleted a bunch of log files, and the df command reported that 3747988 blocks were used and 0 were available. Shouldn't deleting files with the rm command free up disk space?
If I run GParted (under System Tools on the Applications menu), it reports that I have 152.58 MiB available and that this is 4 percent of the partition. Shouldn't this space be available?

The system behavior is weird when memory runs low; after deleting files you should reboot.

You might also consider removing the package files from /var/cache/apt/archives if you have a CD/DVD of the archives. Otherwise you might want to write those files to a CD before removing them (except that you may not be able to if disk space is low - but 'Disk At Once' mode may do).

A certain percentage of a the filesystem is reserved for the root user. The logs were probably owned by root and used up some of this reserve space.

I would look over those logs pretty close. You MAY have something generating an error every second and that can make the logs grow extremely fast. Assuming you are running Ext3 jschiwal is correct, I think the standard default is 5%.

Thank you for your quick responses. I found this answer right after I posted my question, so I'll just see if deleting more files can solve the problem.
This is a pretty stripped down system--it only has a CD-ROM, so I won't be burning disks. I can probably find some more files to delete though.

What do I lose if I delete the files in /var/cache/apt/archives without backing up to CD? Are these just prior versions of the products I've installed? If so, I'm not sure I'd know what to do if I needed them anyway.

"What do I lose if I delete the files in /var/cache/apt/archives without backing up to CD?"

Unless you regularly "clean" after installing software, that directory will contain absolutely all versions of *.deb packages you've installed since first installing the system. For people who use "unstable" and do regular updates, the files can occupy several GB in a fairly short time.

I deleted all the files in /var/cache/apt/archives and this freed up about 30 percent of the partition. Now, booting up and logging in appear to work perfectly! Thank you very much for your help.